Fixed #18210 -- Escaped special characters in reverse prefixes.

Ensured that special characters passed in to reverse via the
prefix argument are properly escaped so that calls to
django.utils.regex_helpers.normalize and/or string formatting
operations don't result in exceptions.

Thanks to toofishes for the error report.

tests/regressiontests/urlpatterns_reverse/tests.py

@@ -171,6 +171,18 @@ class URLPatternReverse(TestCase):
         # Reversing None should raise an error, not return the last un-named view.
         self.assertRaises(NoReverseMatch, reverse, None)
 
+    def test_prefix_braces(self):
+        self.assertEqual('/%7B%7Binvalid%7D%7D/includes/non_path_include/',
+               reverse('non_path_include', prefix='/{{invalid}}/'))
+
+    def test_prefix_parenthesis(self):
+        self.assertEqual('/bogus%29/includes/non_path_include/',
+               reverse('non_path_include', prefix='/bogus)/'))
+
+    def test_prefix_format_char(self):
+        self.assertEqual('/bump%2520map/includes/non_path_include/',
+               reverse('non_path_include', prefix='/bump%20map/'))
+
 class ResolverTests(unittest.TestCase):
     def test_resolver_repr(self):
         """